For the past several decades, the scaling of features in integrated circuits has been a driving force behind an ever-growing semiconductor industry. Scaling to smaller and smaller features enables increased densities of functional units on the limited real estate of semiconductor chips. For example, shrinking transistor size allows for the incorporation of an increased number of memory devices on a chip, lending to the fabrication of products with increased capacity. The drive for ever-more capacity, however, is not without issue. The necessity to optimize the performance of each device becomes increasingly significant.
The operation of spin torque devices is based on the phenomenon of spin transfer torque. If a current of electrons is passed through a magnetization layer, called the fixed magnetic layer, it will come out spin polarized. With the passing of each qualified electron post the tunneling process through the dielectric layer, its spin (which is referred to as “intrinsic” angular momentum of the electron) will impact the magnetization in a next magnetic layer, called the free magnetic layer, causing a small change. Through the principal of conservation of angular momentum, this results in a torque-causing precession of magnetization. Due to reflection of electrons, a torque is also exerted on the magnetization of an associated fixed magnetic layer, but this layer is pinned. In the end, if the current exceeds a certain critical value (given by damping caused by the magnetic material and its environment), the direction of the magnetization of the free magnetic layer will be switched by a pulse of current, typically in less than about 10 nanoseconds. Magnetization of the fixed magnetic layer should remain unchanged since an associated current is below its threshold due to geometry or due to an adjacent anti-ferromagnetic layer.
Spin-transfer torque can be used to flip the active elements in magnetic random access memory. Spin-transfer torque memory, or STTM, has the advantages of lower power consumption and better scalability over conventional magnetic random access memory (MRAM) which uses magnetic fields to flip the active elements. However, significant improvements are still needed in the area of STTM device manufacture and usage.